Railway brake shoes are typically mounted on brake rigging of railway vehicles by means if a brake shoe key, which passes through a longitudinal slot in the brake shoe holder or "brakehead" of the brake rigging and concurrently through mounting holes of the brake shoe backing plate, such holes being located in a central "keybridge" area of the brake shoe backing plate. The keybridge area of the backing plate is generally formed in such a way that it is adapted to protrude into or intersect with a longitudinal slot in the brakehead at or near the center of the brakehead's length. As a result of this centrally located point of attachment and the inherent clearance tolerances that are needed to allow ease of fit, it is possible for the brake shoe to become misaligned relative to the brakehead face when lateral forces are exerted on the sides of the brake shoe. Depending on the location of any such lateral force and its contact location with the brake shoe, the misalignment may be either rotational with the brake shoe rotating at the keybridge so that it is pivoted on the brakehead, or in a parallel sense relative to the length of the brakehead, with the brake shoe displaced linearly relative to the brakehead.
In prior art brake shoe backing plates, various types of lugs or protrusions on the back, convex external surface of the brake shoe backing plate have been employed to guard against this type of misalignment by interlocking such lugs with mating cavities located on the surface of the brakehead. However, the size of these lugs and their longitudinal distance from the central keybridge area are necessarily small relative to the overall length of the brakehead, in order to maintain structural strength of the brakehead sidewalls and to provide as large as possible contact area between the brakehead face and the backing plate of the brake shoe for optimum distribution of normal brake shoe forces. As a result a considerable degree of movement remains possible between the brake shoe and brakehead. When lateral force is applied to the brake shoe it is therefore often possible for misalignment to result, with the effect that the brake shoe is not brought into proper contact alignment with the wheel of the railway car or locomotive. This is detrimental to the proper functioning and structural integrity of the brake shoe, and the wear life of the brake shoe and wheel.
Lug arrangements which overlap the external surfaces of the brakehead rather than interlocking with cavities on the brakehead face, have been proposed in the past, but such proposals have not been successful due either to the need for welding several small pieces to the backing plate (which is difficult to do in a reliable and cost effective manner), or due to weakness of the lugs attached to the outer perimeter of the backing plate for this purpose.